Rubber compositions are often reinforced with reinforcing fillers such as at least one of rubber reinforcing carbon black and synthetic amorphous silica (e.g. precipitated silica).
Various products contain at least one component comprised of such rubber compositions such as, for example, tires.
In order to enhance rubber reinforcing effects of precipitated silica, a coupling agent is typically used in combination with the precipitated silica.
Such coupling agent typically contains a moiety (e.g. alkoxysilane group) reactive with hydroxyl groups (e.g. silanol groups) on the precipitated silica and another different moiety (e.g. polysulfide as a sulfur contributing moiety) interactive with elastomers containing carbon-to-carbon double bonds (e.g. diene-based elastomers).
A typical disadvantage of such polysulfide moiety of the silica coupling agent is its sulfur contribution at an elevated temperature of the uncured rubber composition, such as for example during physical mixing of the uncured rubber composition, which interacts with carbon-to-carbon double bonds of an elastomer in the rubber composition to promote a significantly increased viscosity of the rubber composition which leads to increased rubber processing difficulties, or challenges. Such phenomenon is well known to those having skill in such art.
For this invention, a rubber reinforcing precipitated silica is used which may usually be characterized by having typical properties for a synthetic precipitated silica used for reinforcing rubber compositions such as a BET nitrogen surface area in a range of from 120 to 300 m2/g; a CTAB surface area in a range of from 100 to 300 m2/g, with a ratio of the BET/CTAB surface areas in a range of from 0.8 to 1.3.
The BET nitrogen surface may be determined according to ASTM D1993, or equivalent, and the CTAB surface area according to DIN53535.
A significance of providing the precipitated silica with the required BET/CTAB ratio for rubber reinforcement purposes is exemplified in patent literature as a value in a range of, for example, from 0.8 to 1.3 with a BET surface area of, for example 120 to 300 m2/g and a CTAB surface of, for example, 100 to 300 m2/g. For example, see U.S. patent application Ser. No. 2008/0293871, U.S. Pat. No. 6,013,234 and EP Patent Publication No. 0901986.
The DBP absorption value, or number, may be determined according to DIN 53601 or equivalent.
In one embodiment, such precipitated silica might be typlified, for example, by mercury porosimetric parameters of, for example and as measured by a CARLO-ERBA Porosimeter 2000 or equivalent as a surface area in a range of from 100 to 240 m2/g, and pore size distribution maximum in a range of from 20 to 75 nm (nanometers).
For this invention, it has been discovered that an allyl functionalized rubber reinforcing precipitated silica may be used as silica reinforcement for a rubber composition. Such allyl functionalized silica does not contain a polysulfidic moiety so that sulfur is not available to prematurely interact with the elastomer(s) in the rubber composition. Where the allyl functionalized precipitated silica is formed, for example, by treatment of a precipitated silica with an allyltrialkoxysilane or allylhalosilane, it is envisioned that an alkoxysilane moiety of the allylalkoxysilane or halogen moiety of the allylhalosilane reacts, for example, with hydroxyl groups (e.g. silanol groups) and/or hydrogen groups on the precipitated silica. The treated silica thereby contains allyl hydrocarbon groups which can be interactive with, and thereby couple the silica with diene-based elastomer (elastomer formed by polymerization of monomers containing diene hydrocarbons) in the presence of sulfur curative during the subsequent vulcanization of the rubber composition.
It is further envisioned that, if desired, a beneficially relatively high temperature mixing of the allyl functionalized silica with the elastomer(s) can occur without an attendant sulfur-promoted viscosity increase of the rubber mixture prior to addition of the sulfur and sulfur cure accelerators to the rubber mixture. It is envisioned that such higher temperature mixing of the rubber composition provides an opportunity for more efficient mixing of the rubber composition for a shorter mixing time.
Further, it is envisioned that the presence of allyl functional groups on the silica (e.g. precipitated silica) surface provide a degree of hydrophobicity to the silica surface to thereby enhance, or improve, its dispersion within the rubber mixture with a consequent enhancement, or improvement, in its reinforcement of the rubber composition.
Representative examples of various allyl silanes for preparation of allyl functionalized silica are, for example, allyltriethoxysilane, allyltrimethoxysilane, allyldimethylchlorosilane, allyltrichlorosilane, allylmethyldichlorosilane, diallylchloromethylsilane, diallyldichlorosilane and triallylchlorosilane.
In the description of this invention, the term “phr” relates to parts by weight for a material or ingredient per 100 parts by weight elastomer(s)”. The terms “rubber” and “elastomer” are used interchangeably unless otherwise indicated. The terms “cure” and “vulcanize” are used interchangeably unless otherwise indicated.